Zirconia and mixtures of zirconia with other inorganic oxides are beginning to receive a great deal of attention as ceramic materials. These materials combine toughness with a high tolerance for rapid temperature changes. Because of their unique properties zirconia based materials are already finding application as high temperature furnace insulation, casting dies, pressure nozzles and thread guides. Potential future applications include engine parts, medical prosthesis and electronic materials.
Zirconia (ZrO.sub.2) exists in three polymorphic crystal structures between room temperature and its melting temperature. Pure zirconia is rarely used as a ceramic material. The martensitic tetragonal to ##STR1## monoclinic phase transformation at approximately 1100.degree. C. is diffusionless, athermal and reversible. The 3.25% volume expansion results in catastrophic failure of pure zirconia materials at &gt;1000.degree.C.
Other oxides are added to zirconia to form either fully stabilized zirconia or partially stabilized zirconia. The oxides most commonly used are calcia (CaO), magnesia (MgO) and yttria (Y.sub.2 0.sub.3). Stabilized zirconias result from the addition of enough of these other oxides to form a solid solution with the cubic fluorite structure. Partially stabilized zirconias contain less additives than stabilized zirconias resulting in monoclinic or tetragonal phase zirconia exclusively or as precipitates in the mixed oxide cubic fluorite solid solution. In addition, the temperature of the monoclinic-tetragonal phase transition can be raised to a maximum of approximately 1230.degree. C. by the addition of hafnia to zirconia.
Fully stabilized zirconias have good ion conductivity and can be used as solid electrolytes. However, their high thermal expansion and low thermal conductivity results in poor thermal resistance.
Partially stabilized zirconia typically contains 15-50% unstabilized zirconia crystallites. These materials have good thermal shock resistance. They have low thermal conductivity, low coefficient of friction against steel and good resistance to damage from machining. They often toughen on grinding and sinter to near theoretical density. Unlike fully stabilized zirconia, PSZ has low ionic conductivity and electromagnetic transmissibility.
Zirconium dioxide (zirconia) exists as the naturally occurring minerals baddeleyite and endialite. Zircon (ZrSiO.sub.4 or ZrO.sub.2.SiO.sub.2) also occurs naturally as zirconia silicate (ZrSiO.sub.4) in zircon sand. A variety of methods have been developed for processing zircon sand. The sand can be chlorinated at 1100.degree. C. to produce zirconium chloride: ##STR2## Zircon sand can be converted to zirconia in a plasma process: ##STR3## Pure zirconia (approximately 99%) can be prepared by caustic leaching of silica from the product mixture. Zirconium chloride and zirconium dioxide are the most commonly used precursors for partially stabilized zirconias.
The most energy intensive method of preparing PSZ involves melting and cooling a mixture of the appropriate oxides. This route yields the highest quality PSZ crystals obtained to date.
Zirconia and PSZ powders have been prepared by hydrothermal processes. Single-phase monoclinic zirconia powders have been prepared by hydrothermal treatment of amorphous hydrated zirconia with 8 wt % aqueous KF. The reaction is performed at 100 MPa (14,000 psi), 200.degree.-500.degree. C. for 24 hours to yield 16-22 mm powders. Yttria stabilized cubic zirconia crystals have been prepared hydrothermally at 650.degree.-750.degree. .C and 15,000-22,000 psi.
Yttria stabilized zirconia has been prepared by the hydrolysis of a mixture of zirconium isopropoxide and yttrium isopropoxide. Calcination of the mixed oxide powders is required to obtain the PSZ. PSZ layers can also be obtained from plasma sprayed powders.
PSZ ceramic articles have historically been prepared by sintering mixtures of zirconium oxides with the desired stabilizing oxide. This process generally results in the formation of inhomogenous products with limited strength. It is advantageous to prepare the finished ceramic from a specially prepared PSZ powder.
The usual method of preparing yttria --PSZ powder is the coprecipitation of yttria and zirconia using ammonia. Typical reaction proceeds as follows: ##STR4##
The major problems with this procedure are that the powder obtained is usually not chemically homogeneous and that the powder size is not fine, uniform, or of narrow size distribution.
Partially stabilized zirconias possess a variety of physical properties which make them very attractive for demanding, high technology ceramic products. Their chemical inertness, high strength, thermal stability and tolerance of thermal shocks are responsible for their use in high temperature furnace insulation, casting dies, pressure nozzles and thread guides. Major potential applications include ceramic engine parts and electronic materials.
One of the most attractive potential applications for partially stabilized zirconia is in engine applications. In the conditions of high thermal flux in gas turbines, the thermal stress due to the large coefficient of thermal expansion restricts the use of zirconia to thin coatings or low density insulative parts which can tolerate cracking. The unusual toughness and low thermal conductivity of PSZs makes them more attractive for diesel engine applications where the temperature does not exceed 1000.degree. C.
Zirconia based ceramics are used as sensors in harsh environments such as monitoring automobile exhaust. Zirconia based materials are also used as thermistors and piezoelectric components. Due to the increasingly high demand for partially stabilized zirconias for use in high technology ceramic products, there exists a need for PSZ of very high chemical uniformity having spherical particles of uniform size and submicron diameter. A PSZ having these properties would yield ceramics having a highly pure and uniform density wherein the pores between the spherical particles close uniformly during ceramic formation. Heretofore, such a PSZ has not been readily available. This invention now provides such a PSZ and a process for making such PSZ as well as a FSZ.